Before the chemical characterization of the mammalian hypothalamic GnRH, it was realized that hypothalamic substances regulated production of pituitary LH and FSH. Burgus R., Guillemim R 1970 Hypothalamic releasing factors Ann Rev Biochem 39:499-526. Current contraceptive methods are centered on the existing knowledge of GnRH-gonadotropin-ovarian physiology.
The delineation of mammalian GnRH made possible the ability to create methods to detect and quantify this molecule. The human placenta and the chorionic membranes have also been observed to contain a GnRH-like substance. Gibbons J M, Mitnick M, Chieffo V 1975 In vitro biosynthesis, of TSH- and LH-releasing factors by the human placenta. Am J Obstet Gynecol 121:127-131. The present investigator has recently localized, quantified and demonstrated the synthesis of a GnRH-like substance by the human placenta. Siler-Khodr T M, Khodr G S 1978 Luteinizing hormone releasing factor content of the human placenta. Am J Obstet Gynecol 130:216-219; Khodr G S, Siler-Khodr T M 1978 Localization of luteinizing hormone releasing factor (LRF) in the human placenta. Fert Steril 29:523-526; Siler-Khodr T M, Khodr G S 1979 Extrahypothalamic luteinizing hormone releasing factor (LRF): Release of immunoreactive LRF by the human placenta in vitro. Fert Steril 22:294-296; Khodr G S, Siler-Khodr T M 1980 Placental LRF and its synthesis. Science 207:315-317.
The concentration of immunoreactive GnRH-like material in the placenta and maternal blood has been found to vary with gestational age, following a pattern similar to that of hCG Siler-Khodr T M, Khodr G S, Valenzuela G 1984 Immunoreactive gonadotropin-releasing hormone level in maternal circulation throughout pregnancy. Am J Obstet Gynecol 150:376-379; Sorein K A, Smilde C B, Spencer D K, Yoder B A, Grayson M A, Siler-Khodr T M 1996 Circulating maternal CRH and GnRH in normal and abnormal pregnancies. Am J Obstet Gynecol 175:912-916. It was also demonstrated that exogenous synthetic mammalian GnRH can stimulate hCG production from human placental explants in vitro, and that the GnRH stimulation of hCG release was a receptor mediated event, since it was specific and could be inhibited by a GnRH antagonist, [N-Ac-Pro, D-P-Cl-Phe, D-Nal(2)]-GnRH. Burgus R., Guillemim R 1970 Hypothalamic releasing factors Ann Rev Biochem 39:499-526; Baba Y, Matsui H, Schally A V 1971 Structure of the porcine LH- and FSH-releasing hormone II Confirmation of the proposed structure by conventional sequential analyses Biochem Biophys Res Commun 44:459-463; Gibbons J M, Mitnick M, Chieffo V 1975 In vitro biosynthesis, of TSH- and LH-releasing factors by the human placenta. Am J Obstet Gynecol 121:127-131; Siler-Khodr T M, Khodr G S 1979 Extrahypothalamic luteinizing hormone releasing factor (LRF): Release of immunoreactive LRF by the human placenta in vitro. Fert Steril 22:294-296; Siler-Khodr T M, Khodr G S, Vickery B H, Nestor J J, Jr. 1983 Inhibition of hCG, alpha hCG and progesterone release from human placental tissue in vitro by a GnRH antagonist. Life Sci 32:2741-2745, Khodr G S, Siler-Khodr T M 1979 the effect of luteinizing hormone releasing factor (LRF) on hCG secretion Fert Steril 30:301-304; Siler-Khodr T M, Khodr G S 1981 Dose response analysis of GnRH stimulation of hCG releases from human term placenta. Biol Reprod 25:353-358; Siler-Khodr T M, Khodr G S 1978 Luteinizing hormone releasing factor content of the human placenta. Am J Obstet Gynecol 130:216-21; Khodr G S, Siler-Khodr T M 1978 Localization of luteinizing hormone releasing factor (LRF) in the human placenta. Fert Steril 29:523-526. In addition to the inhibition of hCG, progesterone production was dramatically suppressed. The present investigator also observed that hCG response was related to the gestational age of the placenta. Siler-Khodr T M, Khodr G S, Valenzuela G, Rhode J 1986 Gonadotropin-releasing hormone effects on placental hormones during gestation: 1 Alpha-human chorionic gonadotropin, human chorionic gonadotropin and human_chorionic somatomammotropin. Biol Reprod 34:245-254. In addition, a gestational age-related action of the GnRH antagonist on the release of hCG and steroids was observed. Siler-Khodr T M, Khodr G S, Rhode J, Vickery B H, Nestor J J, Jr. 1987 Gestational age related inhibition of placental hCG, hCG and steroid hormone release in vitro by a GnRH antagonist. Placenta 8:1-14. Further studies demonstrated a potent action of GnRH on placental prostanoids, again resulting in their inhibition when endogenous chorionic GnRH was the highest. Siler-Khodr T M, Khodr G S, Valenzuela G, Harper J, Rhode J 1986 GnRH effects on placental hormones during gestation. 111 Prostaglandin E, prostaglandin F, and 13,14-dihydro-15-keto-prostaglandin F. Biol Reprod 35:312-319; Kang I S, Koong M Y, Forman J S, Siler-Khodr T M 1991 Dose-related action of GnRH on basal prostanoid production from the human term placenta. The 38.sup.th Annual Meeting of the Society for Gynecologic Investigation (San Antonio) Abstract #310:253 (Abstr.). The GnRH antagonist also inhibited basal prostaglandin production with greater potency than equimolar concentrations of GnRH, and this action was partially conserved by mammalian GnRH. Siler-Khodr T M, Khodr G S, Harper M J, Rhode J, Vickery B H, Nestor J J, Jr. 1986 Differential inhibition of human placental prostaglandin release in vitro by a GnRH antagonist. Prostaglandins 31:1003-1010. A chorionic GnRH was identified by the present investigator to regulate hCG in a paracrine fashion within the human placenta. Siler-Khodr T.M. and G. S. Khodr. 1981. The production and activity of placental releasing hormones. In Fetal Endocrinology. J. Resko and W. Montagna, editors. Academic Press Inc. New York. 183-210; Siler-Khodr, T. M. and G. S. Khodr. 1982 GnRH in the placenta. In role of Peptides and Proteins in Control of Reproduction. D. S. Khindsa and S. M. McCann, editors. Elsevier North Holland, New York. 347-363; Siler-Khodr T M 1983 Hypothalamic-like releasing hormones of the placenta. Clin Perinatol 10:533-566; Siler-Khodr T M 1983 Hypothalamic-like peptides of the placenta. Semin Reprod Endocrinol 1:321-333. These data demonstrated that this paracrine axis is of physiologic significance in cell to cell communication, and not of inconsequential, ectopic, tumor production.
Studies of other investigators have reported on the actions of mammalian GnRH on placental function. The chorionic GnRH axis has also been identified as having an observed feedback interaction for activin, inhibit, follistatin, neurotransmitter, prostaglandin and steriods, Shi L Y, Zhang Z W, Li W X 1994 Regulation of human chorionic gonadotropin secretion and messenger ribonucleic acid levels by follistatin in the NUCC-3 choriocarcinoma cell line. Endocrinology 134:2431-2437; Steele G L, Currie W D, Yuen B H, Jia X C, Perlas E, Luang P C 1993 Acute stimulation of human chorionic gonadotropin secretion by recombinant human activin-A in first trimester human trophoblast. Endocrinology 133:297-303; Li W, Olofsson J I, Jeung E B, Krisinger J, Yuen B H, Leung P C 1994 Gonadotropin-releasing hormone (GnRH) and cyclic AMP positively regulate inhibit subunit messenger RNA levels in human placental cells. Life Sci 55:1717-1724; Petraglia F, Vaughan J, Vale W 1991 Inhibin and activin modulate the release of gonadotropin-releasing hormone, human chorionic gonadotropin, and progesterone from cultured human placental cells. Proc Natl Acad Sci U S A 86:5114-5117; Petraglia F, Sawchenko P, Lim A T W, Rivier J, Vale W 1987 Localization, secretion, and action of inhibit in human placenta. Science 237:187-189; Shi C Z, Zhuang L Z 1993 Norepinephrine regulates human chorionic gonadotropin production by first trimester trophoblast tissue in vitro. Placenta 14:683-693; Cemetikic B, Maulik D, Ahmed M S 1992 Opioids regulation of hCG release from trophoblast tissue is mediated by LHRH. Placenta Abstract: 9(Abstr.); Petraglia F, Vaughan J, Vale W 1990 Steroid hormones modulate the release of immunoreactive gonadotropin-releasing hormone from cultured human placental cells. J Chn Endocrinol Metab 70:1173-1178; Haning RV, Jr., Choi L, Kiggens A J, Kuzma D L, Summerville J W 1982 Effects of dibutyryl adenosine 3', 5'-monophosphate, luteinizing hormone-releasing hormone, and aromatase inhibitor on simultaneous outputs of progesterone 17b-estradiol, and human chorionic gonadotropin by term placental explants. J Clin Endocrinol Metab 55:213-218; Petraglia F, Lim A T, Vale W 1987 Adenosine 3', 5-monophosphate, prostaglandin, and epinephrine stimulate the secretion of immunoreactive gonadotropin-releasing hormone from cultured human placental cells. J Clin Endocrinol Metab 65:1020-1025; Harting R V, Jr. Choi L, Kiggens A J, Kuzma D L 1982 Effects of prostaglandin, dibutyryl camp LHRH, estrogen, progesterone, and potassium on output of prostaglandin F2a, 13,14-dihydro-15-keto-prostaglandin F2a, hCG, estradiol, and progesterone by placental minces. Prostaglandins 24:495-506; Barnea E P, Feldman D, Kaplan M 1991. The effect of progesterone upon first trimester trophoblastic cell differentiation and human chorionic gonadotropin secretion. Hum Reprod 6:905-909; Barnea E R, Kaplan M 1989 Spontaneous, gonadotropin-releasing hormone-induced, and progesterone-inhibited pulsatile secretion of human chorionic gonadotropin in the first trimester placenta in vitro. J Clin Endocrinol Metab 69:215-217; Branchaud C, Goodyear C, Lipowski L 1983 Progesterone and estrogen production by placental monolayer cultures: Effect of dehydroepiandrosterone and luteinizing hormone-releasing hormone. J Chn Endocrinol Metab 56:761-766; Ahmed N A, Murphy B E 1988. The effects of various hormones on human chorionic gonadotropin production `in early and late placental explant cultures. Am J Obstet Gynecol 159:1220-1227; Iwashita M, Watanabe M, Adachi T, Ohira A, Shinozaki Y, Takeda Y, Sakamoto S 1989 Effect of gonadal steroids on gonadotropin-releasing hormones stimulated human chorionic gonadotropin release by trophoblast cells. Placenta 10:103-112; Haning R V, Jr., Choi L, Kiggnes A J, Kuzma D L, Summerville J W 1982 Effects of dibutyryl cAMP, LHRH, and aromatase inhibitor on simultaneous outputs of prostaglandin F2a, and 13, 14-dihydro-15-keto-prostaglandin F2a by term placental explants. Prostaglandins 23:29-40; Wilson E, Jawad M 1980 Luteinizing hormone-releasing hormone suppression of human placental progesterone production. Fert Steril 33:91-93. These and other studies established the presence of this paracrine axis, including a negative feedback loop for progesterone and estrogen, similar to that of the hypothalamic-pituitary-gonadal axis. This placental axis, multiple paracrine axes for GnRH and other hypothalamic-like releasing and inhibiting activities have now been defined in the placenta, eye, pancreas, ovary, brain, bone, etc., and are now recognized as essential to normal physiologic functions. Siler-Khodr, T. M. 1992 The Placenta: Part IV-Function of the Human Placenta. In Neonatal and Fetal Medicine. R. A. Polin and W. W. Fox, editors. W. B. Saunders Co. Philadelphia, Pa. 74-86; Youngblood W W, Hurnni J, Kizer J S 1979 TRH-like immunoreactivity in rat pancreas and eye, bovine and sheep ideals, and human placenta: Non-identity with synthetic Pyroglu-His-Pro-NH.sub.2 (TRH). Brain Res 163: 10 1-110; Dubois MP 1975 Immunoreactive somatostatin is present in discrete cells of the endocrine pancreas. Proc Natl Acad Sci USA 72:1340-1343; Adashi. E. Y. 1996. The Ovarian Follicular Apparatus. In Lippincott-Raven Publishers. E. Y. Adashi. J. A. Rock, and Z. Rosenwaks, editors. Lippincott-Raven Publishers, Philadelphia. 17-40.
Recent studies have led to the isolation and characterization of a GnRH gene in the placenta, which is identical to that in the hypothalamus with the exception of the inclusion of the first intron and a very long first exon. Radovick S, Wondisford F E, Nakayama Y, Yamada M, Cutler G B, Jr., Weintraub B D 1990 Isolation and characterization of the human gonadotropin-releasing hormone gene in the hypothalamus and placenta. Mol Endocrinol 4:476-480; Adelman J P, Mason A J, Hayflick J S, Seeburg P H 1986 Isolation of the gene and hypothalamic cDNA for the common precursor of gonadotropin-releasing hormone and prolactin release-inhibiting factor in human and rat. Proc Natl Acad Sci USA 83:179-183; Seebirg P H, Adelman J P 1984 Characterization of cDNA for precursor of human luteinizing hormone releasing hormone. Nature 311:666-668. The message has been localized to the syncytio- and cytotrophoblast, as well as the stroma of the placenta, and is present in higher concentrations during the first half of pregnancy. Duello T M, Tsai S J, Van Ess P J 1993 In situ demonstration and characterization of pro gonadotropin-releasing hormone messenger ribonucleic acid in first trimester human placentas. Endocrinology 133:2617-262-3; Kelly A C, Rodgers A, Dong K W, Barrezueta N X, Blum M, Roberts J L 1991 Gonadotropin-releasing hormone and chorionic gonadotropin gene expression in human placental development DNA Cell Biol 10:411-421. Multiple transcription sites have been identified for the GnRH gene in reproductive tissues, including the placenta. Dong K W, Yu K L, Roberts J L 1993 Identification of a major up-stream transcription start site for the human pro gonadotropin-releasing hormone gene used in reproductive tissues and cell lines. Mol Endocrinol 7:1654-166; Dong K W, Duval P, Zeng Z, Gordon K, Williams R F, Hodgen G D, Jones G, Kerdelhue B, Roberts J L 1996 Multiple transcription start sites for the GnRH gene in rhesus and cynomolgus monkeys: a non-human primate model for studying GnRH gene regulation. Mol Cell Endocrinol 117:121-130; Dong K W, Yu K L, Chen Z G, Chen Y D, Roberts J L 1997 Characterization of multiple promoters directing tissue-specific expression of the human gonadotropin-releasing hormone gene. Endocrinology 138:2754-2762. Steroid regulatory sites on the promoter have also been identified. Chandran U R, Attardi B, Friedman R, Dong K W, Roberts J L, DeFranco D B 1994 Glucocorticoid receptor-mediated repression of gonadotropin-releasing hormone promoter Activity in GTI hypothalamic cell lines. Endocrinology 134:1467-1474; Dong K W, Chen Z G, Cheng K W, Yu K L 1996 Evidence for estrogen receptor-mediated regulation of human gonadotropin-releasing hormone promoter activity in human placental cells. Mol Cell Endocrinol 117:241-246. The functionality of this promoter is supported by showing that GnRH mRNA can be regulated by steroids. Joss J M, King J A, Millar R P 1994 Identification of the molecular forms of and steroid hormone response to gonadotropin-releasing hormone in the Australian lungfish Neoceratodus forsteri. Gen Comp Endocrinol 96:392-400; Montero M, Le Belle N, King J A, Millar R P, Dufour S 1995 Differential regulation of the two forms of gonadotropin-releasing hormone (mGnRH and chorionic GnRH-11) by sex steroids in the European female silver eel (Anguilla anguilla). Neuroendocrinology 61:525-535; Ikeda M, Taga M, Sakakibara H, Minaguchi H, Ginsburg E, Vonderhaar B K 1996 Gene expression of gonadotropin-releasing hormone in early pregnant rat and steroid hormone exposed mouse uteri. J Endocrinol Invest 19:708-713; Gothilf Y, Meiri I, Elizur A, Zohar Y 1997 Preovulatory changes in the levels of three gonadotropin-releasing hormone-encoding messenger ribonucleic acids (mRNSs), gonadotropin. B-submit mRNAs plasma gonadotropin, and steroids in the female gilthead seabream, Spar-us aurata. Biol Reprod 57:1145-1154.
It has previously been accepted that only non-mammalian vertebrates have multiple forms of GnRH in the same species. However, Dellovad, et al. and in 1994, King et al. have described Chicken II GnRH in shew, mole and bat brain, thus demonstrating that two different isomers of GnRH existed in the mammal. Dellovad T L, King J A, Millar R P, Rissman E F 1993 Presence and differential distribution of distinct forms of immunoreactive gonadotropin-releasing hormone in the musk shrew brain. Neuroendocrinology 58:166-177; King J A, Steneveld A A, Curlewis J D, Rissman E F, Millar R P 1994 Identification of chicken GnRH H in brains of inetatherian and early-evolved eutherian species of mammals. Regul Pept 54:467-477. Even then, it was still thought that in modern placental mammalian species, the existence of different GnRHs did not occur. Therefore, the hypothesis of more than one form of GnRH in the human placenta was considered dubious. Chicken II GnRH has now been characterized in the guinea pig and in the human brain. Jimenez-Linan M, Rubin B S, King J C 1997 Examination of guinea pig luteinizing hormone-releasing hormone gene reveals a unique decapeptide and existence of two transcripts in the brain. Endocrinology 13 8:4123-4130; Lescheid D, Terasawa E, Abler L A, Urbanski H F, Warby C M, Millar R P, Sherwood N M 1997 A second form of gonadotropin-releasing hormone (GnRH) with characteristics of chicken GnRH-11 is present in the primate brain. Endocrinology 138:1997. Separate genes for Chicken II GnRH and mammalian GnRH have also been described. White S A, Bond C T, Francis R C, Kasten T L, Fernald R D, Adelman J P 1994 A second gene for gonadotropin-releasing hormone: cDNA and expression pattern in the brain. Proc Natl Acad Sci USA 91:1423-1427; Lin X W, Peter R E 1997 Cloning and expression pattern of a second [His5Trp7Tyr81gonadotropin-releasing hormone (chicken GnRH-H-11) mnRNA in goldfish; evidence for two distinct genes. Gen Comp Endocrinol 107:262-272.
The GnRH in the placenta has not been characterized as fully as the GnRH receptor in the pituitary. Sealfon S C, Weinstein H, Millar R P 1997 Molecular mechanism of ligand interaction with the gonadotropin-releasing hormone receptor. Endocr Rev 18:180-205; Karten M J, Rivier J E 1986 Gonadotropin-releasing hormone analog design. Structure-friction studies toward the development of agonists and antagonists: Rationale and perspective. Endocr Rev 7:44-66 It is known that two populations of placental GnRH receptors exist, one having a Ka of 10.sup.-9 M and the other with a significantly lower affinity of 10.sup.-7 M. In addition, superagonist or antagonist for the pituitary GnRH receptor shows very different affinity for the placental receptor. Escher E, Mackiewicz Z, Lagace G, Lehoux J, Gallo-Payet N, Bellabarba D, Belisle S 1988 Human placental LHRH receptor: Agonist and antagonist labeling produces differences in the size of the non-denatured, solubilize receptor. J Recept Res 8:391-405; Bramley T A, McPhie C A, Menzies G S 1992 Human placental gonadotropin-releasing hormone (GnRH) binding sites: 1 Characterization, properties and ligand specificity. Placenta 12:555-581. Other isomers of GnRH, such as salmon GnRH and Chicken II GnRH, have a much greater affinity for the placental receptor, yet bind with a lesser affinity to the human pituitary receptor. Bramley T A, McPhie C A, Menzies G S 1992 Human placental gonadotropin-releasing hormone (GnRH) binding sites: 1 Characterization, properties and ligand specificity. Placenta 12:555-581. These data demonstrate the existence of a specific placental receptor for GnRH-like molecules, yet the true ligand for this receptor is not known.
In amphibians, a Chicken II GnRH receptor as well as a mammalian GnRH receptor have been shown. The specificity and evolutionary aspects of the GnRH receptor have been studied in many species. Mammalian GnRH has been reported to be active in many vertebrate classes. Other GnRHs, such as Chicken II GnRH and salmon GnRH, have reduced affinity for the mammalian pituitary receptor.
GnRH receptor activity, as well as the mRNA for the GnRH receptor, varies throughout gestation in the human placenta. Bramley T A, McPhie C A, Menzies G S 1994 Human placental gonadotropin-releasing hormone (GnRH) binding sites: 111. Changes in GnRH binding levels with stage of gestation. Placenta 15:733-745; Lin L S, Roberts V J, Yen S S 1997 Expression of human gonadotropin-releasing hormone receptor gene in the placenta and its functional relationship to human chorionic gonadotropin secretion. J Clin Endocrinol Metab 80:580-585. The receptor is greatest in early gestation and appears to be down regulated by 12-20 weeks. While the receptor is again detectable in term placentas, the mRNA (using a GnRH decapeptide probe and in situ hybridization methodology) was undetectable at this state of gestation. Bramley T A, McPhie C A, Menzies G S 1994 Human placental gonadotropin-releasing hormone (GnRH) binding sites: 111. Changes in GnRH binding levels with stage of gestation. Placenta 15:733-745; Lin L S, Roberts V J, Yen S S 1997 Expression of human gonadotropin-releasing hormone receptor gene in the placenta and its functional relationship to human chorionic gonadotropin secretion. J Clin Endocrinol Metab 80:580-585. This pattern of receptor activity is consistent with the concentration of GnRH-like material in placental tissue and maternal blood throughout gestation, and supports the hypothesis that chorionic GnRH may down-regulate its chorionic receptors, as can mammalian GnRH, and its analogs at the pituitary level. Siler-Khodr T M, Khodr G S, Valenzuela G 1984 Immunoreactive gonadotropin-releasing hormone level in maternal circulation throughout pregnancy. Am J Obstet Gynecol 150:376-379; Siler-Khodr T M, Khodr G S 1978 Luteinizing hormone releasing factor content of the human placenta. Am J Obstet Gynecol 130:216-219. Studies by the present investigator and those of Barnea et al, have demonstrated competitive inhibition by GnRH antagonist. Siler-Khodr T M, Khodr G S, Vickery B H, Nestor J J, Jr. 1983 Inhibition of hCG, alpha hCG and progesterone release from human placental tissue in vitro by a GnRH antagonist. Life Sci 32:2741-2745; Siler-Khodr T M, Khodr G S, Harper M J, Rhode J, Vickery B H, Nestor J J, Jr. 1986 Differential inhibition of human placental prostaglandin release in vitro by a GnRH antagonist. Prostaglandins 31:1003-1010; Barnea E R, Kaplan M, Naor Z 1991 Comparative stipulatory effect of gonadotropin releasing hormone (GnRH) and GnRH agonist upon pulsatile human chorionic gonadotropin secretion in superfused placental explants: reversible inhibition by a GnRH antagonist. Hum Reprod 6:1063-1069. Other studies of Szilagyi et al. and Currie et al. indicate that pituitary GnRH agonist can down-regulate the placental GnRH receptor. Szilagyi A, Benz R, Rossmanith W G 1992. The human first-term placenta in vitro: regulation of hCG secretion by GnRH and its antagonist. Gynecol Endocrinol 6:293-300; Currie W D, Setoyarna T, Lee P S, Baimbridge K G, Church J, Yuen B H, Leung P C 1993 Cytosolic free Ca.sup.2 + in human syncytiotrophoblast cells increased by gonadotropin-releasing hormone. Endocrinology 133 :2220-2226. In addition, the demonstration that the placental GnRH receptor can be up regulated in cell cultures by estradiol supports the hypothesis that this receptor is functional in the regulation of placental hormonogenesis. Barnea E R, Kaplan M, Naor Z 1991 Comparative stipulatory effect of gonadotropin releasing hormone (GnRH) and GnRH agonist upon pulsatile human chorionic gonadotropin secretion in superfused placental explants: reversible inhibition by a GnRH antagonist. Hum Reprod 6:1063-1069; Bliatacharya S, Chaudhary J, Das C 1992 Responsiveness to gonadotropin releasing hormone of human term trophoblast cells in vitro: induction by estradiol. Biochein Int 28:363-371.
Another factor that regulates a hormone's activity is its metabolism. The enzyme that degrades GnRH differs during pregnancy from the enzyme that degrades GnRH in the pituitary or the blood of non-pregnant individuals. In placental tissue, the primary enzymatic activity for the degradation of GnRH is chorionic peptidase-1 (C-ase-1), a post-proline peptidase. C-ase-1 is a glycoprotein with a molecular weight of 60,000. Siler-Khodr T M, Kang I S, Jones M A, Harper M J K, Khodr G S, Rhode J 1989 Characterization and purification of a placental protein that inactivates GnRH, TRH and Angiotensin 11. Placenta 10:283-296; Kang I S, Siler-Khodr T M 1992 Chorionic peptidase miactiviates GnRH as a post-proline peptidase. Placenta 13:81-87. It acts as a post-proline peptidase, and is inhibited by bacitracin, para-amino-benzamidine, acetopyruvate and certain cations. Siler-Khodr T M, Kang I S, Jones M A, Harper M J K, Khodr G S, Rhode J 1989 Characterization and purification of a placental protein that inactivates GnRH, TRH and Angiotensin 11. Placenta 10:283-296. GnRH is actively degraded by C-ase-1 at neutral pH, having a Km of 10.sup.-8 M. Kang I S, Gallwitz J, Guzman V, Siler-Khodr T M 1990. Definition of the enzyme kinetics and optimal activity of chorionic peptidase-1. The 23.sup.rd Annual Meeting of the Society for the Study of Reproduction (Vancouver) (Abstract #311):144(Abstr.). Using immunofluorescent methodology, C-ase-1 has been localized by the present inventor in the cytoplasm of the syncytiotrophoblast and syncytial buds. It is secreted into maternal blood, where GnRH is not stable without specific inhibitors of this post-proline peptidase. Benuck M, Marka N 1976 Differences in the degradation of hypothalamic releasing factors by rat and human serum. Life Sci 19:1271-1276. C-ase-1 is present in very high concentrations, and accounts for virtually all GnRH degrading activity in the placenta under physiological conditions.
These in vitro studies support the hypothesis of the specific, receptor-mediated and enzyme-regulated action of mammalian GnRH on placental hormonogenesis, and demonstrate the paracrine effects and feedback interactions for numerous intrauterine hormones interacting with chorionic GnRH. Further studies on the action of mammalian GnRH and its analogs in vivo have also demonstrated these paracrine interactions for chorionic GnRH-like activity and numerous other chorionic hormones, and have established the physiologic role of GnRH in the maintenance of normal pregnancy. Siler-Khodr, T. M. 1993. Luteinizing Hormone Releasing Hormone (LHRH) and the Placenta and Fetal Membranes. In Molecular Aspects of Placental and Fetal Membrane Autocoids. G. E. Rice and S. P. Brennecke, editors. CRC Press, Inc. Ann Arbor. 339-350; Petraglia F, Calza L, Garuti G C, Giardino L, De Ramundo B M, Angioni S 1990. New aspects of placental endocrinology. J Endocrinol Invest 65:262-267.
Recent studies demonstrate that the number of GnRH receptors and mRNA for the GnRH receptor in the placenta varies in a pattern similar to that of hCG.Duello T M, Tsai S J, Van Ess P J 1993. In situ demonstration and characterization of pro gonadotropin-releasing hormone messenger ribonucleic acid in first trimester human placentas. Endocrinology 133:2617-2623; Lin L S, Roberts V J, Yen S S 1997. Expression of human gonadotropin-releasing hormone receptor gene in the placenta and its functional relationship to human chorionic gonadotropin secretion. J Clin Endocrinol Metab 80:580-585. Other investigators have shown steroid responsive elements in the placental GnRH gene, providing further evidence for the physiologic regulation of placental GnRH-like activity. Dong K W, Chen Z G, Cheng K W, Yu K L 1996 Evidence for estrogen receptor-mediated regulation of human gonadotropin-releasing hormone promoter activity in human placental cells. Mol Cell Endocrinol 117:241-246. Petraglia et al. has described the pulsatile release of a GnRH-like substance, which has a specific pulse frequency, amplitude and duration, with increased amplitude during early gestation. Petraglia F, Genazzani A D, Aguzzoli L, Gallinelli A, de Vita D, Caruso A, Genazzani A R 1994. Pulsatile fluctuations of plasma-gonadotropin-releasing hormone and corticotropin-releasing factor levels in healthy pregnant women. Acta Obstet Gynecol Scand 73:284-289. Other investigators using Rhesus monkey embryos have demonstrated the secretion of a GnRH-like substance by the peri-implantation embryo, which precedes the secretion of chorionic gonadotropin. Seshagiri P B, Terasawa E, Heam J P 1994. The secretion of gonadotropin-releasing hormone by peri-implantation embryos of the rhesus monkey: comparison with the secretion of chorionic gonadotropin. Hum Reprod 9:1300-1307.
Other investigators have shown that administration of high doses of mammalian GnRH, its agonistic analogs or antibodies, to pregnant baboons and monkeys effects a sharp decrease of CG production and progesterone, which in most cases leads to termination of pregnancy. Gupta S K, Singh M 1985 Characteristics and bioefficacy of monoclonal antigonadotropin releasing hormone antibody. Am J. Repro Immunol Microbiol 7:104-108; Das C, Gupta S K, Talwar G P 1985 Pregnancy interfering action of LHRH and anti-LHRH. J. Steroid Biochem 23:803-806; Hodges J K, Hearn J P 1977 Effects of immunization against luteinizing hormone releasing hormone on reproduction of the marmoset monkey Callithrix jacchus. Nature 265:746-748; Vickery B H, McRae G I, Stevens V C 1981 Suppression of luteal and placental function in pregnant baboons with agonist analogs of luteinizing hormone-releasing hormones. Fert Steril 36:664-668; Das C, Talwar G P 1983 Pregnancy-terminating action of a luteinizing hormone-releasing hormone agonist D-Ser(But)6desGlylOProEA in baboons. Fert Steril 39:218-223; Rao A, Moudgal N 1984 Effect of LHRH injection on serum chorionic: gonadotropin levels in the pregnant bonnet monkey (Macaca radiata). Obstet Gynecol 12:1105-1106; Rao A J, Chakraborti R, Kotagi S G, Ravindranath N, Moudgal N R 1985 Effect of LHRH agonists and antagonists in male and female bonnet monkeys (Macaca Radiata). J. Steroid Biochem 23:807-809. Interruption of pregnancy was most consistently observed when these mammalian GnRH analogs were administered following implantation. In pregnant women, administration of low doses of mammalian GnRH does not significantly change circulating hCG. Tamada T, Akabori A, Konuma S, Araki S 1976 Lack of release of human chorionic gonadotropin by gonadotropin-releasing hormone. Endocrinol Jpn 23:531-533; Perez-Lopez F R, Robert J, Teijeiro J 1984 Prl, TSH, FSH, B-hCG and oestriol responses to repetitive (triple) LRH/TRH administration in the third trimester of human pregnancy. Acta Endocrinol 106:400-404. However, this finding was dose and gestational age related. Egged J, Gati 1 1985 Elevated serum hCG level after intravenous LH-RH administration in human pregnancies. Endocrinol Exp 19:11-15; Iwashita M, Kudo Y, Shinozaki Y, Takeda Y 1993 Gonadotropin-releasing hormone increases serum human chorionic gonadotropin in pregnant women. Endocrine Journal 40:539-544.
A recent study of Devreker et al. reports that the use of long-acting mammalian GnRH analogs in IVF, impaired the implantation rate. Devreker F, Govaerts 1, Bertrand E, Van den Bergh M, Gervy C, Englert Y 1996. The long-acting gonadotropin-releasing hormone analogues impaired the implantation rate. Fert Steril 65:122-126. While these analogs have proven to be generally nontoxic, long-term chronic use has been associated with a hypo-estrogenic state. Accidental administration of mammalian GnRH analogs during early pregnancy has been reported, with varied outcomes. Siler-Khodr, T. M. 1994. Potentials for embryo damage of GnRH analogs. In Ovulation Induction: Basic Science and Clinical Advances. M. Filicor and C. Flamigni, editors Elsevier Science B. V. Amsterdam. 279-306. Generally, pregnancy outcomes appeared unaffected, but increased cases of spontaneous abortion and pre-term labors have also been observed. The varied outcomes may reflect the different doses and protocols of administration of these mammalian GnRH analogs, as well as the different analogs employed. For analogs that can be rapidly metabolized by the chorionic tissues, little effect, if any, would be anticipated. In addition, the affinity for the placental receptor for many of these mammalian GnRH analogs is greatly reduced as compared to the pituitary receptor's affinity. In those case, little chorionic effect would be observed.